This invention relates to the technique of ultrasonic echoscopy of objects and in particular to an extension of known techniques of ultrasonic echoscopy to provide more useful information concerning the examined objects. It is particularly, but not solely, directed to the more effective acquisition of data in medical diagnosis utilising this technique.
Ultrasonic echoscopy provides information about an examined object which may be displayed in the form of an ultrasonic echogram. Such an echogram consists of a display of acoustic impedance discontinuities or reflecting surfaces in the object. It is obtained by directing a short pulse of ultrasonic energy, typically in the 1-30 MH z frequency range, into the examined object where any acoustic impedance discontinuities in the object reflect and return some of the energy in the form of an echo. This echo is received, converted into an electric signal and displayed as an echogram on a cathode ray oscilloscope, a film, a chart or the like.
The echogram may constitute either a one dimensional or a two dimensional representation and in both cases the information is contained in the position and magnitude of the echo displayed. In a one dimensional display, the position along a base line is used to indicate the distance to the reflecting surface whilst the magnitude of the echo is displayed, for example, as a deflection of the base line or as an intensity change. In a two dimensional display, the position along a base line is used to indicate the distance to the reflecting surface as in a one dimensional display, and the direction of the base line is used to represent the direction of propagation of the acoustic energy. The two dimensional display is obtained by changing this direction of propagation of the acoustic energy and by instituting a similar but not necessarily identical movement of the base line of the display. The magnitude of the echo is displayed as for a one dimensional display; for example, as a deflection of the base line or as an intensity change.
The technique of ultrasonic echoscopy is used in medical diagnosis to obtain information about the anatomy of patients. The application of the technique is now widely investigated and is described, for example, by D. E. Robinson in Proceeding of the Institution of Radio and Electronics Engineers Australia, Vol. 31, No. 11, pages 385-392, November, 1970; "The Application of Ultrasound in Medical Diagnosis". As pointed out in this article, ultrasonic echoscopy may be used to produce displays resembling anatomical cross-sections which have proved clinically useful when the desired information concerns physical dimensions, shapes of organs or structures or the like. Ultrasonic echography has proved of particular value as a diagnostic aid in the abdomen and pregnant uterus, eye, breast, brain, lung, kidney, liver and heart, these being areas of soft tissue with little bone and air. In general, the technique is considered to complement other techniques to provide a more complete picture of the patients condition, however particularly in pregnancies, ultrasonic echoscopy may be useful in place of X-rays where the latter may not give sufficient information or may be dangerous. In medical use, a pulse of ultrasonic energy is transmitted into a patient in a known direction and echoes are received from reflecting surfaces within the body. The time delay between a transmitted pulse and the received echo depends on the distance from the transmitter to the reflecting surface and the distance information so obtained may be displayed in a suitable way for interpretation and clinical use as a one dimensional range reading or as a two dimensional cross section as previously described.
If a pulse of ultrasound is propagated into a medium, echoes will be received at various time delays and these time delays will be proportional to the distances from the transducer producing the pulse to the interfaces provided the velocity of propagation is constant. In soft tissues found in the human body the velocity of sound is reasonably constant and pulsed ultrasound provides a convenient method of measuring the depth of a particular structure from the transducer face without inconvenience to the patient. This information can be used in a number of ways.
In the simplest form of display, "A mode", the echoes are presented as deflections of the trace of an oscilloscope in which distance is represented along the time axis. This mode is useful clinically when the source of the various echoes displayed can be positively identified. It is possible to measure the distance between two echoes, or between the energising pulse and an echo, with accuracy but it may not be possible to identify the source of the echoes. It has been used to measure the size of the baby's head inside the uterus, the depth of the eye and the bladder and to locate the mid-line in the brain. Similar information may be displayed by use of the "B mode" in which the echoes are presented as a brightening or intensity modulation of the time-base trace.
If the interface of interest is moving, its position may be plotted with time ("M mode") by using the B mode presentation and allowing the time base to be swept at right angles to its direction so as to display the movements of the interface echo backwards and forwards along the time base. This is used to demonstrate the pulsatile movements of various parts of the heart and brain. If the B mode is used but the trace on the screen is made to represent the line of sight of the transducer and then the transducer is scanned around the patient and the time base line on the screen made to follow, a two-dimensional plot of impedance discontinuities is obtained. Two dimensional visualisation has been used in the pregnant uterus, abdomen, eye and breast.
Coupling from the transducer to the patient may be achieved by skin contact or by use of a water delay bath. If a water delay bath is used the distance between the transducer and the skin surface must be greater than the largest depth of penetration to be used, to avoid ambiguity due to multiple reflection. In general the skin contact scan results in greater comfort for the patient and echograms of less clarity while the water delay scan gives less patient comfort and better quality echograms.
Focusing of transducers used in ultrasonic echoscopy is known to reduce the width of the ultrasonic beams generated by the transducer and thus improve the lateral resolution. Known methods of reducing the width of an ultrasonic beam by focusing or shaping the beam include (i) focusing with a lens or a mirror in a manner similar to focusing in optics, (ii) use of a curved transducer, and (iii) use of a multi-element transducer (known as a phased array) in which the elements of the transducer are energized at progressively different times to generate the desired shape of the beam.